1. Field of the Invention
The present invention relates to a method for the production of photographic images in accordance with the DTR (silver halide diffusion transfer) method and to a processing apparatus for carrying out this method.
2. Description of the Prior Art
The principles of the photographic silver complex diffusion transfer reversal process, herein called DTR-process wherein DTR stands for diffusion transfer reversal, have been described e.g. in U.S. Pat. No. 2,352,014.
In the DTR-process, silver complexes are image-wise transferred by diffusion from an image-wise exposed and developed silver halide emulsion layer to an image-receiving layer, where they are converted into a silver image. For this purpose, an image-wise exposed silver halide emulsion layer is developed by means of a developing substance in the presence of a so-called silver halide solvent. In the exposed parts of the silver halide emulsion layer, the silver halide is developed to silver so that it cannot dissolve anymore and thus cannot diffuse. In the non-exposed parts of the silver halide emulsion layer, the silver halide is converted into soluble silver complexes by means of a silver halide complexing agent (a so-called silver halide solvent) and transferred by diffusion into an image receiving layer arranged or kept in waterpermeable relationship with the silver halide emulsion layer to form in the image-receiving layer, usually in the presence of development nuclei, a silver metal image. More details on the DTR-process can be found in the book "Photographic Silver Halide Diffusion Processes" by A. Rott and E. Weyde, Focal Press, London, New York (1972).
In a particular DTR-system, the image-receiving layer forms an integral part of the photographic material, in other words forms a mono-sheet system. In said mono-sheet system, the image-receiving layer is separated from the silver halide emulsion layer by an opaque but waterpermeable layer. The opaque layer precludes the detection by the eye of a silver image formed in the silver halide emulsion layer. Examples of opaque pigment layers suited for the above purpose are described in DE-B1 1,961,030, DE-A1 1,772,603, in BE-A1 526,587, in GB-A 878,064 and in U.S. Pat. No. 3,928,037.
The processing of image-wise exposed DTR-type materials occurs almost universally by conveying the exposed light-sensitive sheet along a concavely curved path through a body of processing liquid contained in a trough. At its exit a roller pair for pulling the sheet(s) at a uniform speed through the liquid and for squeezing the sheet(s), thus removing most of the liquid in order to enable a quick drying of the sheet(s), is provided. In the case of a two-sheet system, the outlet roller pair also ensures the tight contact between both sheets to enable the transfer of the soluble silver complexes from the light-sensitive towards the image-receiving sheet.
Apparatus for carrying out the described process are simple and rugged of construction and reliable from a mechanical point of view. They comprise a trough for the processing liquid, the bottom wall of this trough being concavely curved to serve as a sheet guide, a sloping entry guide plate for guiding the sheet towards the deepest point of the trough and a pair of outlet rollers for pulling the sheet at a uniform speed through the apparatus and for squeezing liquid from it.
The consistency of processing results obtained with these known apparatus leaves much to be desired, since it has been shown that after a period of use of approximately one week, the processing liquid of the apparatus has deteriorated such that the quality of the processed images becomes unsatisfactory. This deterioration is caused by different factors. First, the absorption by the liquid in the trough of CO.sub.2 from the air. Next, evaporation which causes the concentration of the processing liquid to change and, finally, oxidation to the air. So the processing liquid in the apparatus has to be replaced at least after approximately one week, independent of the number of developments carried out. This replacement is not only time consuming and expensive, but also produces a large number of empty bottles from the fresh processing liquid and a serious amount of unusable processing liquid to be destroyed in an ecologically acceptable way.
It has been proposed to overcome deterioration of the processing liquid by providing a rigid cover on the free surface of the liquid in the trough and by compensating liquid carried off by processed sheets through a replenishing system, such as a barometric feeding arrangement. Such system is disclosed in U.S. Pat. No. 3,148,611.
The operation of this system is critical since the slightest deviation of the apparatus from a truely horizontal position breaks the contact of the surface cover with the free liquid surface in the trough. Liquid leaking onto the cover remains in place and deteriorates on the air. Finally displacements or tilting of the apparatus can produce air bubbles remaining captured under the cover.
Photographic processors are known in which floats are used to reduce the free surface of the processing liquid. However, such processors have separate storage tanks with parallel walls so that varying liquid levels do not impede the proper working of the floats as they are free to go up and down with the liquid in the tank.